This application relates to transesterification. More particularly, this application relates to a continuous process of base-catalyzed transesterification. Our invention is the use of certain metal oxide solid solutions, and the layered double hydroxides from which they originate, as the basic catalyst means to effect a continuous process of transesterification.
Transesterification is a chemical reaction proceeding according to equation 1. ##STR1## where P is reactant ester, Q is reactant alcohol, Y is the product ester, Z is the product alcohol and R.sub.1, R.sub.2 and R.sub.3 are organic radicals. Transesterification may be acid catalyzed or base catalyzed. Depending upon the nature of R.sub.1 and R.sub.3 acid catalysis can lead to appreciable side reactions, for example, skeletal isomerization and olefin formation. Therefore base catalyzed transesterification often is favored over acid catalyzed transesterification. Where a continuous catalytic process is desired the catalyst often is used as a fixed bed. The desirability of a strong base suitable for use as a fixed bed previously has been recognized and has led to the use of materials such as, inter alia, sodium on alumina and potassium on graphite. Because of the severe limitations of such strong bases in a fixed bed, more recent attention has turned to clays and clay-like materials as suitable alternatives.
Hydrotalcite is a clay with the ideal unit cell formula of Mg.sub.6 Al.sub.2 (OH).sub.16 (CO.sub.3.4H.sub.2 O, and closely related analogs with variable magnesium/aluminum ratios may be readily prepared. Nakatsuka et al., Bull. Chem. Soc. Japan, 52, 2449 (1979) has described the catalytic use of "calcined synthetic hydrotalcite", i.e., synthesized hydrotalcite calcined at 450.degree. C. prior to use, with varying molar ratios of MgO/Al.sub.2 O.sub.3 in the batch mode polymerization of beta-propiolactone. More extensive work was reported later on the use of "synthetic hydrotalcite" in various base-catalyzed reactions by W. T. Reichle, J. of Catalysis, 94, 547 (1985), who found that aldol condensations in a pulse reactor were readily catalyzed by "synthetic hydrotalcite" compositions calcined at 450.degree. C. having Mg/Al ratios from 1.3 to 6.3, although the Mg/Al ratio did not appear to have a significant effect on either its catalytic activity or efficiency. From deuterium exchange studies Reichle also concluded that the pK.sub.a of hydrotalcite was between 35 and 45. E. Suzuki and Y. Ono, Bull. Chem. Soc. Japan, 61, 1008 (1988), reported on the aldol condensation between formaldehyde and acetone using as catalysts two quite different types of hydrotalcite-like materials generally calcined at 500.degree. C., both being derived from hydrotalcite itself. In one series of catalysts the carbonate moiety of hydrotalcite was exchanged by NO.sub.3.sup.-, SO.sub.4.sup.2-, or CrO.sub.4.sup.2-, and in the other series there was isomorphous substitution of Mg.sup.2+ --Al.sup.3+ by Li.sup.+ --Al.sup.3+, Co.sup.2+ -Al.sup.3+, Ni.sup.2+ --Al.sup.3+, or Zn.sup.2+ --Cr.sup.3+. At 500.degree. C. reaction temperature none of the foregoing appeared to lead to increased acetone conversion although some slight increase in selectivity (especially at lower conversion) was observed. Nunan et al., J. of Catalysis, 116, 222 (1989), has prepared related materials by isomorphous substitution of Mg by Cu and Zn, and of Al by Cr or Ga.
Before proceeding it appears advisable to prevent semantic obfuscation by defining several terms, using first a specific example and then generalizing by analogy. Although "hydrotalcite" is most properly applied to a clay of composition Mg.sub.6 Al.sub.2 (OH).sub.16 (CO.sub.3).4H.sub.2 O often it has been used to describe related layered double hydroxides with varying Mg/Al ratios. However, after calcination of the layered double hydroxides the resulting materials are better described as solid solutions of magnesium oxide and aluminum oxide with the formula Mg.sub.6 Al.sub.2 O.sub.8 (OH).sub.2. That is, calcination destroys the layered structure characteristic of hydrotalcite and affords a solid solution. But the terminology as applied to such solid solutions often retains the "hydrotalcite" name, as in, for example, "synthetic hydrotalcites". In this application henceforth we shall try to be consistent in using the term "solid solution" of, e.g., magnesium oxide and aluminum oxide, to describe such calcined synthetic materials. The second point involves the use of the term "Mg/Al". In this application Mg/Al shall be the number ratio of magnesium to aluminum atoms in a solid solution of magnesium oxide and aluminum oxide. While this definition has been previously employed by, for example, Reichle, others have used a different definition for the Mg/Al ratio.
We can generalize the foregoing characterization to the family of layered double hydroxides having the general formula A.sub.a B.sub.b (OH).sub.(2a+2b) (X.sup.-n).sub.b/n.zH.sub.2 O where A is a divalent metal or combination of divalent metals, B is a trivalent metal or a combination of trivalent metals, a and b are relative number of atoms of A and B, respectively, and X is an anion, often carbonate. After calcination of the layered double hydroxide the resulting product is a solid solution of the two oxides with the formula A.sub.a B.sub.b O.sub.(a+b) (OH).sub.b. We shall retain this distinction between layered double hydroxides and solid solutions throughout this application.
Our objective was the development of a continuous process for the transesterification of esters generally. It was important that the process be continuous and employ a fixed bed of catalyst. Therefore the catalyst had to possess suitable flow properties, compressibility, and so forth, consistent with a liquid flow. It also is important that the transesterification proceed in relatively high yield relative to thermodynamic equilibrium, with good selectivity, and at modest temperatures, say less than 300.degree. C. Since alcohols are both a reactant product and may also be employed in large excess as a solvent, it is important that the catalysts exhibit stability in the presence of polar, hydroxylic material. We have found that solid solutions of one or more divalent metal oxides and one or more trivalent metal oxides with high surface area appear to satisfy the foregoing criteria in all respects. We also have found that their corresponding layered double hydroxides also may be utilized as a base catalyst in a continuous process of transesterification.